1. Field of the Invention
This invention relates to polyolefin film, e.g., polyethylene, polypropylene, polybutene and the like, coated on one side with a heat sealable composition of an acrylic interpolymer and coated on the other side with a heat sealable composition of polyvinylidene chloride copolymer, which is suitable for packaging a variety of items, e.g., foodstuffs.
2. Description of the Prior Art
In the preparation of films useful for packaging purposes, typically separate plies of acrylic coated polyolefin film, e.g., biaxially oriented polypropylene film, have been combined with a PVDC (polyvinylidene chloride) coated cellophane or PVDC coated glassine using conventional adhesive laminating or thermal laminating techniques for application in modern horizontal and vertical form, fill, and seal machines.
The acrylic coated film ply, normally utilized as the outside of the package, displays excellent hot slip and jaw release characteristics, but comparatively poor hot tack or melt strength characteristics. The PVDC coated film ply normally used on the inside displays excellent hot tack, but poor jaw release characteristics and high hot COF (coefficient of friction). Consequently, placement of the acrylic coated ply on the outside, puts the acrylic coating in direct contact with the sealer surfaces where its good hot slip and jaw release characteristics are most essential.
The PVDC coating is normally placed on the inside where it provides good hot tack characteristics where most needed. Hot tack is a characteristics of the coating which is releated to the melt strength of the coating itself, and is specifically the tendency of the films to remain sealed when subjected to a peeling stress while still hot. Additionally, the PVDC coated ply provides a barrier to transmission of gases such as oxygen thus improving the shelf life of the packaged product.
However, with respect to such multiple ply film, problems with hot tack and concomitantly sealing strength are found typically in the aforementioned vertical form, fill and seal applications where a relatively heavy product is being packaged. In standard high speed equipment, the bottom sealer opens immediately after filling is completed, thus subjecting the bottom of the package to the full weight of the product packaged before the partially melted coating in the seal area has hardened. This results in goods dropping out the bottom of the bag.